I teach a course on agile and most of the people has many troubles with user stories in general. It is normal! Many authors are talking about technical stories or other kind of stories and that means they also don’t really feel comfortable with user stories. Well, I usually think in user stories as something that the stakeholders can check on a demo. Something the user/stakeholder can consider an improvement, they can feel satisfied or happy about it. If you cannot show it, or you need to enter in weird technical details they don’t care/understand, it is not an user story, it is an expected task or task or whatever. So for defining an user story remember you need to think first how that should be demonstrated. It is at that moment when you can guess in an easier way if it is an user story or not.
Also, there are some problems when gathering user stories in a bigger one.
But that will be another day since I have to present now!